Conventionally, an electromagnetic valve, which is one type of solenoid devices, has a plunger, which is movably provided in a center hole of an electromagnetic coil in the axial direction thereof. A valve body is provided at the front end of the plunger. There is shown a proportional control valve for controlling the amount of flow in FIG. 15.
A proportional control valve 900 has an electromagnetic coil 904, which covers the outer face of a plunger 902. There are provided yokes 906 and 908 around the electromagnetic coil to enclose the magnetic circuit. The plunger 902 is biased to the electromagnetic coil 904 by leaf springs 910 and 912. The force working on the plunger 902 is transmitted to a valve body 916 by a slide pin 914.
Upon passing the electric current through the electro-magnetic coil 904, the magnetic force corresponding to the intensity of electric current, voltage or number of pulse biases the plunger 902 downward to press the valve body 916 which is connected to a diaphragm. The amount of flow at an exit 920 can be controlled under the governor theory.
An example of displacement sensors is disclosed as Japanese Patent Provisional Publication (Kokai) No. 61-172011. This displacement sensor has a coil and a diaphragm. A magnetizable body, which can be slidable in the coil, is attached to the diaphragm. The end face of the magnetizable body is arranged outside of the coil. The change of the reactance (or the frequency) of the coil caused by displacement of the magnetizable body which is connected to the diaphragm is measured as the amount (or pressure) of displacement of the diaphragm.